Method and apparatus for integrated cable drop compensation of a power converter

ABSTRACT

An integrated circuit controller includes a switching control circuit coupled to output a drive signal to control switching of a switch to regulate an output power of a power converter to be coupled to a distribution network. An oscillator is coupled to output a clock signal coupled to be received by the switching control circuit. The switching control circuit is coupled to output the drive signal in response to the clock signal. A cable drop compensator is coupled to output a compensated reference voltage signal coupled to be received by the switching control circuit in response to a switching signal from the switching control circuit. The switching signal is representative of the drive signal. The cable drop compensator is coupled to adjust the compensated reference voltage signal proportionately to a load current to compensate for a distribution voltage across the distribution network.

RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.13/232,867, filed Sep. 14, 2011, now pending, which is a divisional ofU.S. patent application Ser. No. 12/058,526, filed Mar. 28, 2008, nowissued as U.S. Pat. No. 8,035,254, which claims the benefit of U.S.Provisional Patent Application No. 60/922,193, filed Apr. 6, 2007. U.S.application Ser. No. 13/232,867 and 60/922,193 and U.S. Pat. No.8,035,254 are hereby incorporated by reference.

BACKGROUND INFORMATION

1. Field of the Disclosure

The present invention relates generally to power converters, and morespecifically, the invention relates to voltage regulation of powerconverters.

2. Background

Many electrical devices such as cell phones, personal digital assistants(PDA's), laptops, etc. are powered by a source of relatively low-voltageDC power. Because power is generally delivered through a wall outlet ashigh-voltage AC power, a device, typically referred to as a powerconverter, is required to transform the high-voltage AC power tolow-voltage DC power. The low-voltage DC power may be provided by thepower converter directly to the device or it may be used to charge arechargeable battery that, in turn, provides energy to the device, butwhich requires charging once stored energy is drained. Typically, thebattery is charged with a battery charger that includes a powerconverter that meets constant current and constant voltage requirementsrequired by the battery. Other electrical devices, such as DVD players,computer monitors, TVs and the like, also require a power converter fordevice operation. The power converter in these devices also has toprovide output voltages and currents that meet the requirements of thedevice. In operation, a power converter may use a controller to regulateoutput power delivered to an electrical device, such as a battery, thatmay be generally referred to as a load. More specifically, thecontroller may be coupled to a sensor that provides feedback informationof the output of the power converter in order to regulate powerdelivered to the load. The controller regulates power to the load bycontrolling a power switch to turn on and off in response to thefeedback information from the sensor to transfer energy pulses to theoutput from a source of input power such as a power line. One particulartype of power converter that may be used is a flyback power converter.In a flyback power converter, an energy transfer element maygalvanically isolate the input side of the power converter from theoutput side. Galvanic isolation prevents DC current from flowing betweenthe input side and the output side of the power converter, and isusually required to meet safety regulations.

Power converter control circuits may be used for a multitude of purposesand applications. There is a demand for integrating control circuitfunctionality that can reduce the number of components outside theintegrated control circuit. This reduction in external component countenables miniaturization of the power converter to improve portability,reduces the number of design cycles required to finalize a powerconverter design and also improves reliability of the end product.Furthermore, reduced component count can offer energy efficiencyimprovements in the operation of the power converter and can reduce thepower converter cost. Typically, a power converter has special circuitson the output side of the power converter to sense and to transmitfeedback information about the output voltage to the control circuitthat is on the input side of the power converter. One technique toreduce the number of components in the power converter is to sense thefeedback information of the output voltage from the input side of thepower converter instead of sensing it on the output side of the powerconverter. This is accomplished by a means of an indirect feedback. Onechallenge associated with power converters using indirect feedback iscompensating for the varying voltage dropped across a cable thatconnects the power converter (e.g. battery charger) to the load(battery). Indirect feedback can regulate the voltage at the output ofthe power converter that is at one end of the cable, but the voltage theother end of the cable will be different from the voltage at the outputof the power converter by the voltage drop of the cable. By compensatingfor the additional voltage drop of the cable, the power converterprovides improved voltage regulation at the load.

There are known discrete circuits that are implemented externally to anintegrated power supply controller, which can compensate for the voltagedrop of the cable. However, the known discrete circuits that compensatefor the voltage drop across the cable introduce additional componentsthat increase the cost and size of the power converter. For example,known discrete cable drop compensation circuits may include relativelylarge capacitors that increase the size of the power converter. Inaddition, known discrete circuits that compensate for voltage dropacross the cable may not be suitable for certain power converters usingcontrollers that implement certain advanced control methods.

BRIEF DESCRIPTION OF THE DRAWINGS

Non-limiting and non-exhaustive embodiments of the present invention aredescribed with reference to the following figures, wherein likereference numerals refer to like parts throughout the various viewsunless otherwise specified.

FIG. 1 is a schematic illustrating generally an example functional blockdiagram of a power converter coupled to a distribution network and loadincluding an example controller in accordance with the teachings of thepresent invention.

FIG. 2A is a functional block diagram further illustrating generally theexample controller of FIG. 1 in accordance with the teachings of thepresent invention.

FIG. 2B illustrates generally example waveforms of signals correspondingwith the example controller of FIG. 2A in accordance with the teachingsof the present invention.

FIG. 3A is a functional block diagram further illustrating generally theexample cable drop compensator of FIG. 2 in accordance with theteachings of the present invention.

FIG. 3B illustrates generally example waveforms of signals correspondingwith the example cable drop compensator of FIG. 3A in accordance withthe teachings of the present invention.

FIG. 3C illustrates generally alternate example waveforms of signalscorresponding with the example cable drop compensator of FIG. 3A inaccordance with the teachings of the present invention.

FIG. 4 is a flow chart illustrating generally an example method forcable drop compensation for a power converter in accordance with theteachings of the present invention.

FIG. 5 is a flow chart illustrating generally an alternate example of amethod for cable drop compensation for a power converter in accordancewith the teachings of the present invention.

FIG. 6 is a schematic illustrating generally an example power convertercoupled to a distribution network including an example controller inaccordance with the teachings of the present invention.

FIG. 7 is a schematic illustrating an example of an integrated cabledrop compensator in accordance with the teachings of the presentinvention.

FIG. 8 is a schematic illustrating an alternative example of anintegrated cable drop compensator that accommodates for varying currentlimits in accordance with the teachings of the present invention.

DETAILED DESCRIPTION

Examples related to integrated cable drop compensation circuits andmethods for use in power converters are disclosed. In the followingdescription, numerous specific details are set forth in order to providea thorough understanding of the present invention. It will be apparent,however, to one having ordinary skill in the art that the specificdetail need not be employed to practice the present invention.Well-known methods related to the implementation have not been describedin detail in order to avoid obscuring the present invention.

Reference throughout this specification to “one embodiment,” “anembodiment,” “one example” or “an example” means that a particularfeature, structure or characteristic described in connection with theembodiment is included in at least one embodiment or example of thepresent invention. Thus, the appearances of the phrases “in oneembodiment,” “in an embodiment,” “in one example” or “in an example” invarious places throughout this specification are not necessarily allreferring to the same embodiment. The particular features, structures orcharacteristics may be combined for example into any suitablecombinations and/or sub-combinations in one or more embodiments orexamples. Furthermore, the particular features, structures orcharacteristics may be included in an integrated circuit, an electroniccircuit, a combinational logic circuit, or other suitable componentsthat provide the described functionality.

As will be discussed, example power converters in accordance with theteachings of the present invention include an integrated cable dropcompensation feature that eliminates discrete components, and alsoextends usability to power converters for advanced control methods. Onecontrol method referred to herein is termed “on/off” control. “On/off”herein refers to whether or not the power switch is enabled to conduct.An “on” cycle is one wherein the switch is enabled, and therefore, mayconduct for a portion of time during that cycle, and an “off” cycle isone wherein the switch is disabled, or prevented from conducting. Thus,“on/off” in the present disclosure does not refer to whether the switchis, in fact, conducting in a given cycle, only whether or not switchconduction is enabled. Another control method referred to as pulse widthmodulation herein is termed “PWM.” More specifically, PWM involvesmodulating the on time, also referred to as the conduction time, of theswitch during a cycle that may be of fixed duration or variableduration. Another form of PWM involves modulating the off time, which iswhen the switch is prevented from conducting, of the switch during acycle that may be of fixed duration or variable duration. It isappreciated that modulating the on time is indistinguishable frommodulating the off time when the switching cycles are of fixed duration.Examples of the disclosed power converters and methods may be used in avariety of applications in which the input side of the power converteris galvanically isolated from the output side and a load voltage isregulated in response to sensing a signal at the input siderepresentative of the output voltage of the power converter.

To illustrate, FIG. 1 shows generally an example power converter 100coupled to a distribution network 102 and a load 104, including anexample controller in accordance with the teachings of the presentinvention. As shown, the distribution network 102 is coupled to anoutput 105 that corresponds with an output voltage V_(OUT) 106. A loadcurrent I_(LOAD) 109 flows from output 105 and through the distributionnetwork 102 to supply the load 104. Typically, the distribution network102 includes a cable, such as for example a power cord. In one example,the distribution network 102 may include, but is not limited to,terminal interfaces and/or any other elements that provide additionalresistance at the output side of the power converter 100. The terminalinterfaces may include for example a connection interface between thepower supply and the cable as well as a connection interface between thecable and the load.

As shown, input 110 corresponds with an input voltage VIN 112 and iscoupled to an energy transfer element 114, which in the illustratedexample provides galvanic isolation between input 110 and output 105. Inother words, the energy transfer element 114 prevents DC current fromflowing from the input side to the output side of the power converter100. In one example, energy transfer element 114 includes an inputwinding 116 and an output winding 117. An “input winding” may also bereferred to as a “primary winding” and an “output winding” may also bereferred to as a “secondary winding.” As shown in the illustratedexample, a clamp circuit 118 is coupled across input winding 116 ofenergy transfer element 114 to limit the voltage across a controller119.

As shown, the controller 119 is coupled to input 110 and coupled toregulate output 105. In various examples, controller 119 may includefeatures to employ any of a variety of switching schemes including butnot limited to, on/off control, on/off control with varying currentlimit levels, variable or constant frequency pulse width modulation(PWM), or the like. The controller 119 is also coupled to a sensor 120that senses a value of an output 105 of the power converter 100. In theexample, output voltage V_(OUT) 106 is sensed by sensor 120. Asillustrated in the example, output voltage V_(OUT) 106 and the sensor120 are separated by a rectifier 140 and by the magnetic couplingbetween two windings 116 and 117 of an energy transfer element 114 thatin the example is a transformer. The sensor 120 is coupled to output afeedback signal 124 U_(FB) to the controller 119 in order to regulateoutput 105.

In operation, controller 119 regulates the output 105 of power converter100 by switching a power switch included in controller 119 in responseto the feedback signal 124 U_(FB). When the power switch in controller119 is on, energy from the input 110 is transferred into the inputwinding 116 of the energy transfer element 114 and is stored in theenergy transfer element 114. When the power switch in controller 119 isoff, the energy stored in the energy transfer element 114 is transferredto the output winding 117. The energy from output winding 117 istransferred to the output 105 of the power converter 100 with apulsating current that flows through a forward biased rectifier 140 toan output capacitor 142. A substantially DC (non-pulsating) load current109 flows from the output 105 of the power converter through adistribution network 102 to a load 104. An input return 125 is coupledto input terminals 110 and an output return 127 is coupled to outputterminals 105. In one example, the input return 125 and output return127 may be coupled.

As shown, a load current I_(LOAD) 109 is delivered through thedistribution network 102 to supply the load 104. In one example, theload 104 may represent a device to be charged, such as for example acell phone battery and may require a regulated load voltage V_(LOAD)130. In operation, controller 119 produces pulsating current in theoutput winding 117. The current in the output winding 117 is rectifiedby the rectifier 140 and is filtered by output capacitor 142 to producethe substantially constant output voltage V_(OUT) 106. As the loadcurrent I_(LOAD) 109 increases, a distribution voltage drop V_(DIST) 132increases in proportion due to a substantially constant resistance inthe distribution network 102. In one example, the power converter 100may compensate for the distribution voltage drop V_(DIST) 132 to receivebetter regulation at the load 104 by controlling the output voltageV_(OUT) 106 in accordance with the teachings of the present invention.Since the resistance of the distribution network 102 may be differentfor different applications, it will be appreciated that a user must knowand account for the approximate resistance of the particulardistribution network in choosing appropriate feedback components inorder to realize the benefits of the invention.

In one example, the power converter 100 may operate in a discontinuousconduction mode (DCM), where all energy from the energy transfer element114 is transferred to the output winding 117 before the input winding116 receives more energy from the input 110.

In accordance with the teachings of the present invention, the variationof load voltage V_(LOAD) 130 is reduced as compared to prior powerconverters. This is accomplished by adjusting the output voltage V_(OUT)106 to compensate for a changing voltage drop (V_(DIST) 132) across thedistribution network 102. As described above, the distribution voltagedrop V_(DIST) 132 will change in response to the current delivered tothe load 104. The invention teaches that adjusting the output voltageV_(OUT) 106 to compensate for varying distribution voltage V_(DIST) 132may be accomplished in response to the switching of a power switch inthe controller 119.

FIG. 2A is a functional block diagram 200 illustrating generally furtheraspects of an example of the controller 119 of a power converter 100 inaccordance with the teachings of the present invention. As shown in theillustrated example, controller 119 further includes a power switch 208,a switching block 201, an integrated cable drop compensator 202, anoscillator 204, and a current limiter 206. In the example of FIG. 2A thecontroller 119 may represent the boundary of an integrated circuit thatincludes power switch 208, switching block 201, integrated cable dropcompensator 202, and current limiter 206, in a single monolithic device.In other examples one or more of these functional blocks may beimplemented with discrete circuit components, monolithic integratedcircuits, hybrid integrated circuits or various combinations thereof.

In the depicted example, switching block 201 regulates output voltage106 by switching the power switch 208 that is between a first terminal209 and a second terminal 210. In one example, first terminal 209 may bereferred to as a drain terminal and second terminal 210 may be referredto as a source terminal. In operation, the power switch 208 may be in anon state, which allows current flow through power switch 208, or in anoff state, which substantially prevents current flow through the powerswitch 208. A drive signal 211 from switching block 201 controls theswitching of the power switch 208 to regulate an output at output 105 ofpower converter 100. A switching signal 212 is representative of drivesignal 211 and is output to cable drop compensator 202. In one example,when switching block 201 is using a constant frequency or variablefrequency PWM control scheme, the switching signal 212 may represent‘on’ times and ‘off’ times of power switch 208.

In another example, when switching block 201 is using an on/off controlscheme, the switching signal 212 may represent ‘enabled’ cycles and‘disabled’ cycles. In an enabled cycle, the switch may conduct, and in adisabled cycle, the switch is prevented from conducting. It is notedthat “disabled cycles” may also be referred to as “skipped cycles.”During an on/off control scheme, switching of the power switch 208operates based on cycles of fixed duration as shown in FIG. 2B. Usingon/off control, the switching block 201 may regulate the output voltageV_(OUT) 106 by deciding to execute a switching event during a cycle orby skipping a switching event during a cycle. More specifically, aswitching event is defined as when the power switch 208 transitions froman off state to an on state and back to an off state in a given cycle.The duration of the conduction time of the power switch 208 in anenabled cycle may be of a fixed duration or a variable duration and theduration may be determined any number of ways including by a timingsignal of a fixed duration, a current limit, or the like. As shown inthe example, when a switching event occurs, the cycle is enabled (EN)and the switching signal 212 is high for the entire duration of thecycle. When a switching event is skipped, the cycle is disabled (DIS)and the switching signal 212 is low for the duration of the cycle.

Continuing with the example shown in FIG. 2A, switching block 201receives feedback signal 124 substantially representative of an outputvoltage V_(OUT) 106 via a feedback terminal 213. The switching block 201switches the power switch 208 in response to feedback signal 124. Asshown, the integrated cable drop compensator 202 outputs a compensatedreference voltage signal 216 that is representative of an adjustedvoltage V_(ADJ) which is representative of the value of output voltageV_(OUT) that is required to compensate for a distribution voltageV_(DIST) 132. The output voltage V_(OUT) 106, which is regulated inresponse to the adjusted reference voltage value V_(ADJ), issubstantially the sum of a distribution voltage V_(DIST) 132 and a loadvoltage V_(LOAD) 130. When the load current 109 is high, a relativelyhigh distribution voltage drop 132 will be present, and when the loadcurrent 109 is low, a relatively low distribution voltage drop 132 willbe present. Therefore, improved regulation of load voltage V_(LOAD) 130is accomplished when compensated reference voltage signal 216 adjustsproportionately to load current 109 to compensate for a distributionvoltage drop 132.

As shown in the depicted example, the switching block 201 receives aclock signal 214 from the oscillator 204. The clock signal 214 is apulsating signal used as a time reference by switching block 201 forswitching power switch 208. For example, in an example of on/offcontrol, the clock signal 214 would be referenced to maintain aswitching cycle of fixed duration. In an example of PWM control, theclock signal 214 would be referenced for controlling the on time or offtime of the power switch 208 for each cycle.

As shown, a current limiter 206 is coupled to a current sense 217 thatsenses a switch current I_(SWITCH) 218. In the illustrated example,current sense 217 is coupled to detect switch current I_(SWITCH) 218between power switch 208 and second terminal 210. In another example, itis appreciated that current sense 217 may be coupled to detect switchcurrent I_(SWITCH) 218 between power switch 208 and first terminal 209.The current limiter 206 outputs a current limit reached signal 220 tothe switching block 201 when switch current I_(SWITCH) 218 reaches apeak current limit I_(PEAK). The switching block 201 may change the peakcurrent limit I_(PEAK) and output a peak current limit adjust signalI_(ADJ) 222. More specifically, adjusting the peak current limitI_(PEAK) may be based on or responsive to operating parameters such as,but are not limited to, mode of operation and control scheme. In oneexample, a maximum peak current limit I_(MAX) is a value that limits themaximum value of switch current I_(SWITCH) 218. In one example, peakcurrent limit adjust signal I_(ADJ) 222 is received by cable dropcompensator 202 to determine a current limit ratio K_(lRATIO). In oneexample, the current limit ratio K_(lRATIO), may be used to control theswitching of power switch 208. The current limit ratio K_(lRATIO) may becalculated by the following equation:

$\begin{matrix}{K_{IRATIO} = \frac{I_{PEAK}}{I_{MAX}}} & (1)\end{matrix}$

FIG. 3A is a functional block diagram 300 further illustrating anexample integrated cable drop compensator 202 of FIG. 2A according tothe teachings of the present invention. The integrated cable dropcompensator 202 includes a switching coefficient calculator 302 and avoltage compensation calculator 304. As shown, switching coefficientcalculator 302 outputs a switching coefficient signal 306 representativeof a switching coefficient value (SC). More specifically, the switchingcoefficient is representative of the switching of a power switch in amanner that is representative of the ratio of load current 109 dividedby the maximum amount of deliverable load current 109. For example, whenthe switching coefficient SC is 1.0, then maximum load current 109 isbeing delivered to the load, which results in a maximum voltage dropacross the distribution network 132. When switching coefficient SC is0.5, half of maximum load current 109 is being delivered to load 104,which results in half of the maximum voltage drop across thedistribution network 132.

FIG. 3B illustrates an example calculation of a switching coefficient SCwhen switching block 201 is using an on/off control scheme in accordancewith the teachings of the present invention. As shown in FIG. 3B, theswitching signal 212 indicates an enabled cycle (EN) when a switchingevent occurs during that cycle and indicates a disabled cycle (DIS) whena switching event does not take place. In an on/off control scheme, theswitching coefficient SC may be calculated by determining the number ofcycles the switching signal 212 is high out of a total number of cycles.In one example, the switching coefficient may be calculated with thefollowing equation:

$\begin{matrix}{{SC} = {\frac{N_{ENABLE}}{N_{ENABLE} + N_{DISABLE}} = \frac{N_{ENABLE}}{N_{TOTAL}}}} & (2)\end{matrix}$where N_(ENABLE) is defined as the number of enabled cycles andN_(DISABLE) is defined as the number of disabled cycles. A total numberof switching cycles N_(TOTAL) is defined as the sum of enabled anddisabled cycles within a set time. It will be apparent when calculatingthe switching coefficient in a discrete manner, the number of switchingcycles to be considered must be large enough to provide a substantiallyconstant value for the switching coefficient SC when the load current isconstant, and yet must be small enough to maintain the voltage at theload within specified limits when the load current changes.

To illustrate, FIG. 3B shows an example in which the number of enabledcycles N_(ENABLE) is equal to eight, the number of disabled cyclesN_(DISABLE) is equal to 12, and total number of cycles N_(TOTAL) isequal to 20. The switching coefficient SC is determined by dividing thenumber of enabled cycles N_(ENABLE) by the total number of cyclesN_(TOTAL) to get a switching coefficient SC of 0.4.

FIG. 3C illustrates an alternate example calculation of a switchingcoefficient SC with an example switching block 201 using a variablefrequency pulse width modulation (PWM) control scheme in accordance withthe teachings of the present invention. As shown, the switching signal212 is a digital signal that is high when power switch 208 is in an onstate, and is low when power switch 208 is in an off state. In theillustrated PWM control scheme, the switching coefficient SC may becalculated by determining the effective duty ratio of the switchingsignal over a period of time. More specifically, the switchingcoefficient may be determined with the following equation:

$\begin{matrix}{{SC} = {\frac{t_{ON}}{t_{ON} + t_{OFF}} = \frac{t_{ON}}{t_{TOTAL}}}} & (3)\end{matrix}$where t_(ON) is defined as the time power switch 208 is on, which iswhen current is allowed to flow, and t_(OFF) is defined as the timewhere switch remains off, within a total time t_(TOTAL). As shown in theexample of FIG. 3C, the on time t_(ON) is equal to 100 μs, the off timet_(OFF) is equal to 150 μs, and the total time t_(TOTAL) is equal to 250μs. The switching coefficient SC is calculated by dividing the on timet_(ON) 100 μs by the total time t_(TOTAL) 250 μs, which results in aswitching coefficient SC of 0.4 in the illustrated example.

Continuing with the example cable drop compensator 202 in FIG. 3A, thevoltage compensation calculator 304 outputs the compensated referencevoltage signal 216 in response to switching coefficient signal 306. Theswitching coefficient SC is a fraction that represents the value of theload current divided by the maximum load current. Therefore, theswitching coefficient signal 306 indicates the required amount ofcompensation as a fraction of the maximum expected distribution voltagedrop V_(DIST) 132. The compensated reference voltage signal 216increases the output voltage V_(OUT) 106 above its value at no loadcurrent by an amount that is substantially the same fraction of themaximum expected distribution voltage drop V_(DIST) 132. Thus, theoutput voltage V_(OUT) 106 compensates for the distribution voltage dropV_(DIST) 132 that varies with the load current 109. In one example,further described with reference to FIG. 7, the switching coefficientmay be calculated continuously instead of calculated in a discretefashion as described above in FIGS. 3B and 3C in accordance with theteachings of the present invention.

FIG. 4 is a flow chart 400 illustrating generally an example method forintegrated cable drop compensation in accordance with the teachings ofthe present invention. As shown in the illustrated example, processingbegins at block 405, and in block 410, switching signal 212 is received.In block 420, a switching coefficient in response to switching signal212 is determined. In block 430, an adjusted voltage V_(ADJ) iscalculated in response to the switching coefficient SC and a referencevoltage V_(REF). The reference voltage V_(REF) is an internal voltagewithin the integrated circuit that remains substantially constant over arange of external conditions, and is set based on design parameters ofthe power converter 100. The adjusted voltage V_(ADJ) is substantiallythe same value as V_(REF) when the switching coefficient SC is zero. Inblock 440, output voltage V_(OUT) 120 is regulated to keep the feedbacksignal 124 at the adjusted voltage V_(ADJ) in order for V_(OUT) 106 tocompensate for distribution voltage V_(DIST) 132 and limit the varianceof load voltage V_(LOAD) 130. Processing then returns to block 410.

FIG. 5 is a flow chart 500 illustrating generally another example forintegrated cable drop compensation in accordance with the teachings ofthe present invention. As shown, flow chart 500 of FIG. 5 shares somesimilarities with flow chart 400 of FIG. 4. For instance, processingbegins in block 405. In block 410, the switching signal 212 is received.In block 420, a switching coefficient SC is determined in response toswitching signal 212. However, in block 523, it is determined whether ornot the current limit has changed in the present cycle from a previouscycle in response to a current adjust signal I_(ADJ) 222. If it isdetermined in block 523 that current limit has changed, then currentlimit ratio K_(lRATIO) is calculated in block 525 and the adjustedvoltage V_(ADJ) is calculated in response to reference voltage V_(REF),current limit ratio K_(lRATIO), and switching coefficient SC in block527. In the discontinuous conduction mode of operation, output power isproportional to the square of the peak current limit I_(PEAK).Therefore, in one example the adjusted voltage V_(ADJ) may be calculatedin response to the square of the current limit ratio (K_(lRATIO))².

If it is determined in block 523 that current limit has not changed,then an adjusted voltage V_(ADJ) is calculated in block 430 in responseto reference voltage V_(REF) and switching coefficient SC. In block 440,output voltage V_(OUT) 106 is regulated to keep the feedback signal 124at the adjusted voltage V_(ADJ) in order for V_(OUT) 106 to compensatefor distribution voltage V_(DIST) 132 and limit variance of load voltageV_(LOAD) 130. Processing then returns to block 410.

FIG. 6 is an example schematic 600 illustrating generally a powerconverter 100 coupled to a distribution network 102 including an examplecontroller in accordance with the teachings of the present invention. Asshown, the energy transfer element 114 in conjunction with an integratedcontroller 602 is coupled to regulate output voltage V_(OUT) 106 atoutput terminals 105 of power converter 600. In one example, integratedcontroller 602 implements the functions of controller 119 shown inFIG. 1. The integrated controller 602 is coupled between drain terminal209 and source terminal 210. The feedback terminal 213 is coupled to theintegrated controller 602 to receive feedback signal 124 representativeof output voltage V_(OUT) 106. In the illustrated example, bypassterminal 604 is coupled to a bypass capacitor 606, which provides supplycurrent to the internal circuitry of integrated controller 602 duringoperation. In one example, clamp circuit 118 includes a resistor 605, arectifier 606, and a capacitor 607 to limit maximum voltage acrossintegrated controller 602.

In the example, FIG. 6 illustrates sensor 120 further including anauxiliary winding 608 that outputs feedback signal 124 coupled to bereceived by integrated controller 602. In one example, the voltageappearing across auxiliary winding 608 is substantially proportional tothe output voltage V_(OUT) 106 of the converter during a time afterpower switch 208 is turned off. In addition, auxiliary winding 608 iscoupled to a voltage divider that includes first and second resistors612 and 614 such that feedback terminal 213 is coupled between first andsecond resistors 612 and 614. In one example, values for first andsecond resistors 612 and 614 may be chosen based on the desired level ofload voltage V_(LOAD) 130 at a given load current, accounting for thevoltage on the rectifier 140 when it is conducting. In other examplesusing a PWM control method, it is appreciated that the supply currentfor integrated controller 602 to operate may be derived from theauxiliary winding 608.

FIG. 7 is a schematic of a circuit 700 illustrating an exampleintegrated cable drop compensator 202 in accordance with the teachingsof the present invention. As shown, an internal voltage source 701 iscoupled to supply a regulated internal voltage V_(INT) 702 for circuit700. In one example, internal regulated voltage V_(INT) 702 may besupplied by the bypass capacitor 606 via bypass terminal 604, asillustrated in the example of FIG. 6.

As shown in the depicted example, a switching signal 212, which isrepresentative of drive signal 211, is received by an inverter 710. Inone example, switching signal 212 may be a digital signal that is activeduring the duration of an enabled cycle and inactive otherwise. In oneexample, a logical high digital signal represents an active signal and alogical low signal represents an inactive signal. When switching signal212 is high, the gate of a transistor T₂ 714 will be low allowingcurrent to flow through resistor R₁ 716 and resistor R₂ 718. Whenswitching signal 212 is low, the gate of transistor T₂ 714 will be highpreventing current flow through transistor T₂ 714 and R₂ 718.

In the illustrated example, a current source 715 is coupled to atransistor T₁ 725 to sink current from the internal voltage supply 701.As shown, a low pass filter 720 is coupled to a node A 721, a node B 724which is coupled to the gate of a transistor T₃ 722, and a common return737. In one example, the low pass filter includes a capacitor 727 and aresistor 729 coupled as shown. In operation, when the gate of transistorT₂ 714 is high, a voltage drop V_(R1) 731 across resistor R₁ 716 issubstantially zero due to a relatively high resistance of resistor 729with respect to the resistance of resistor R₁ 716.

As shown, a first graph 733 a illustrates an AC voltage waveformrepresentative of a voltage drop V_(R1) 731. More specifically, thevoltage drop with a magnitude of V_(R1MAX) is dropped across resistor R₁716 when gate of transistor T₂ 714 is low and a voltage drop ofsubstantially zero volts is dropped across resistor R₁ 716 when gate oftransistor T₂ 714 is high. A second graph 733 b illustrates a DC voltagewaveform representative of voltage drop V_(R3) 735 across a resistor R₃726. More specifically, the voltage drop V_(R3) is a continuouslyaveraged value of AC voltage drop V_(R1) 731. In other words, low passfilter 720 is coupled to provide a substantially DC voltage V_(R3) 735across resistor R₃ 726 from a time-varying voltage V_(R1) 731.Transistors 725 and 722 are sized to operate at low current density.Consequently, the voltage between gate and source of transistor 725 isapproximately the same as the voltage between gate and source oftransistor 722. Therefore, in the example shown, low pass filter 720effectively functions as the switching coefficient calculator 302. Inone example, V_(R1) has the value V_(R1MAX) when the switching signal212 is high, and V_(R1) is substantially zero when the switching signal212 is low or in the “disabled state.” Therefore, the average voltageV_(R3) in N switching cycles is V_(R1MAX) divided by N and multiplied bythe number of enabled cycles in the N switching cycles.

As shown, a voltage compensation calculator 730 includes matchedresistors R₄ 732 and R₅ 734 coupled with matched in size transistors T₄736 and T₅ 738, respectively. A current source 740, coupled to internalvoltage supply V_(INT) 702, supplies current I to the voltagecompensation calculator 730. In the illustrated example, a secondcurrent source 742 is coupled to common return 737 and conducts half ofthe current of current source 740.

In one example, when almost all cycles are disabled, the switchingcoefficient is substantially equal to zero and current throughtransistor T₃ 722 is substantially equal to zero. Furthermore, thecurrent through a transistor T₄ 736 and a resistor R₄ 732 will be thesame as current through a transistor T₅ 738 and resistor a R₅ 734.Therefore, an adjusted voltage V_(ADJ) 746 will be substantially thesame value as a fixed reference voltage V_(REF) 744.

In another example, the switching coefficient SC is 0.5 or 50%, whichfor example may indicate that power switch 208 is enabled for 50% of aset number of cycles. During the time when the switching signal 212 islow, substantially no current flows through resistors R₁ 716 and R₂ 718.Conversely, when switching signal 212 is high, current flows throughresistors R₁ 716 and R₂ 718, and there will be a voltage drop acrossresistor 716. The voltage drop may be determined by the followingequation:

$\begin{matrix}{V_{R\; 1} = {( \frac{R_{1}}{R_{1} + R_{2}} ) \times ( {V_{INT} - V_{{GS}\; 1} - V_{{GS}\; 2}} )}} & (4)\end{matrix}$where V_(R1) is the voltage drop across resistor R₁, V_(INT) 702 is theinternal supply voltage 702, V_(GS1) is the gate to source voltageassociated with transistor 725, and V_(GS2) is the drain to sourcevoltage associated with transistor 714.

In operation, the voltage at a node C 741 with respect to common return737 adjusts in response to the additional current flowing throughresistor R₃ 726 and resistor R₄ 732. More specifically, when currentflow increases through resistor R₃ 726 the voltage drop across resistorR₄ increases which raises the voltage at node C 741 with respect tocommon reference 737. When current flow decreases through R₃ 726 thevoltage drop across resistor R₄ decreases which reduces the voltage atnode C 741 with respect to common reference 737. Since the voltageacross resistor R₅ 734 is held constant by current source 742, and thevoltage at node C 741 is changing, the adjusted voltage V_(ADJ) 746changes by the same amount as the voltage at node C. According to thisimplementation, adjusted voltage V_(ADJ) 746 may be representative ofthe following equation:V _(ADJ) =V _(REF) +V _(R4) −V _(R5)  (5)where V_(R4) is the voltage drop across resistor R₄ 732 and V_(R5) is afixed voltage drop across resistor R₅ 735, V_(REF) 744 is a setreference voltage with respect to common return 737.

In one example, a comparator 750 included in the switching block 201receives the compensated reference voltage signal 216 representative ofadjusted voltage V_(ADJ) 746 and compares it to feedback signal 124representative of output voltage V_(OUT) 106 to regulate output voltageV_(OUT) 106 to compensate for a changing distribution voltage V_(DIST)132 to maintain limited variance of load voltage V_(LOAD) 130.

FIG. 8 is a schematic illustrating a modified example of an integratedcable drop compensator in FIG. 7 that accommodates for varying peakcurrent limits in accordance with the teachings of the presentinvention. As shown, a resistor R₆ 801 and a transistor T₆ 802 areincluded and are coupled to allow more than one current limit. In theillustrated example, the current adjust signal I_(ADJ) 222 is receivedby cable drop compensator 202 via a current limit terminal 804. In oneexample, the current adjust signal I_(ADJ) 222 may be an analog signalto accommodate multiple current limit levels. In another example,multiple signals may designate multiple current limit levels through theuse of multiple resistors between resistor R₂ 718 and R₆ 801, eachadditional resistor to be shorted by a transistor that receives acurrent adjust signal, such that either the transistor or the resistorconducts the current in resistor R₂ 718. According to the exampleillustrated in FIG. 8, current adjust signal I_(ADJ) 222 is a digitalsignal that that turns transistor 802 between an ‘off’ state and an ‘on’state. More specifically, when current adjust signal I_(ADJ) 222 ishigh, resistor R₆ is shorted and the current limit is at the maximumpeak current limit I_(MAX). The voltage across resistor R₁ 716 can becalculated the same way as discussed with respect to FIG. 7 usingEquation (4) above.

When current adjust signal I_(ADJ) 222 is low, present peak currentlimit I_(PEAK) is reduced from maximum peak current limit I_(MAX) andtransistor 802 is ‘off’ resulting in a reduced voltage drop acrossresistor R₁. The voltage across resistor 716 R₁ may be calculated basedon the following equation:

$\begin{matrix}{V_{R\; 1} = {( \frac{R_{1}}{R_{1} + R_{2} + R_{6}} ) \times ( {V_{INT} - V_{{GS}\; 1} - V_{{GS}\; 2}} )}} & (6)\end{matrix}$where V_(R1) is the voltage drop across resistor R₁ 716, V_(INT) is theinternal supply voltage 702, V_(GS1) is the gate to source voltageassociated with transistor T₁ and V_(GS2) is the drain to source voltageassociated with transistor T₂. As shown in the equation above, voltagedrop V_(R1) 731 decreases due to the additional resistance of resistorR₆ 801. More specifically, the V_(R1) 731 is decreased in proportion tothe decrease in peak current limit I_(PEAK) from the max peak currentlimit I_(MAX). Therefore, the adjusted voltage V_(ADJ) 746 will belimited by the same proportion the V_(R1) value in Equation (6) has beenreduced from the V_(R1) value in Equation (4). In one example, multiplepeak current limits may be added and implemented using the principlesexplained above in accordance with the teachings of the presentinvention.

In the foregoing detailed description, the method and apparatus of thepresent invention have been described with reference to specificexamples or embodiments thereof. It will, however, be evident thatvarious modifications and changes may be made thereto without departingfrom the broader spirit and scope of the present invention. The presentspecification and figures are accordingly to be regarded as illustrativerather than restrictive.

What is claimed is:
 1. An integrated circuit controller for a powerconverter to regulate an output power of the power converter to becoupled to a distribution network, comprising: a switching controlcircuit coupled to output a drive signal to control switching of aswitch to regulate the output power of the power converter; anoscillator coupled to output a clock signal coupled to be received bythe switching control circuit, wherein the switching control circuit iscoupled to output the drive signal in response to the clock signal; anda cable drop compensator coupled to output a compensated referencevoltage signal coupled to be received by the switching control circuitin response to a switching signal from the switching control circuit,wherein the switching signal is representative of the drive signal,wherein the cable drop compensator is coupled to adjust the compensatedreference voltage signal proportionately to a load current to compensatefor a distribution voltage across the distribution network.
 2. Theintegrated circuit controller of claim 1, further comprising a currentlimiter coupled to output a current limit reached signal in response toa switch current to flow through the power switch, wherein the switchingof the switch is responsive to the compensated reference voltage signal,a feedback signal and the current limit reached signal.
 3. Theintegrated circuit controller of claim 2, wherein the switching controlcircuit and the cable drop compensator are included in a singlemonolithic device, and wherein the switch is included in the singlemonolithic device.
 4. The integrated circuit controller of claim 3wherein the cable drop compensator comprises: a switching coefficientcalculator coupled to output a switching coefficient signal in responseto the switching signal from the switching control circuit; and avoltage compensation calculator coupled to output the compensatedreference voltage signal in response to the switching coefficientsignal.
 5. The integrated circuit controller of claim 4 wherein theswitching coefficient calculator comprises a low pass filter coupled togenerate the switching coefficient signal.